The present invention relates to a method and apparatus for processing holographic interference patterns using Fourier transforms.
Numerous methods have been suggested for the processing of holographic interference patterns. These methods are briefly discussed and appropriate cross-reference is made to materials listed in the bibliography section of the application.
Measurements of deformation fields by holographic interferometry require the determination of interference phase from the fringe patterns. From the phase distribution together with the sensitivity vectors, which are given by the geometrical arrangement of the optical components, the displacement vectors and thus the deformations are determined..sup.1-2, 16-17 Every quantitative measurement using holographic interferometry has to take into account the speckle noise and nonuniform background-intensity variations. The accurate measurements needed for strain and stress analysis require the determination of the interference phase, even between the fringe maxima and minima. This may be achieved by numerical interpolation after locating the centers of dark and bright fringes..sup.3 More precise results are obtained by the evaluation of three or four interferograms reconstructed with mutual phase shifts in the reference wave..sup.4-8 The phase shifts may be performed during real time reconstruction.sup.5-7 or by the double-exposure technique with the application of two-reference-beam holography..sup.4,8 If the phase shift has a known value, three reconstructions suffice,.sup.7,8 but the use of four reconstructions with an unknown but constant value, offers advantages in error-reduction capability..sup.4-6
Nonlinear regression analysis has been proposed for determining the phase distribution from one distributed intensity distribution of a holographic interferogram along one line..sup.9 Since each individual irradiance measurement carries information about the interference phase at its position, this method uses all information available to smooth and interpolate the phase of the fringes.
Automated holographic interference fringe evaluation normally uses digital image-processing methods..sup.3,5,6,10,11 The intensities of the fringe pattern are recorded by a vidicon or charge-coupled-device camera along a rectangular array of pixels and are quantized to discrete values normally ranging from 0 to 255, corresponding to 8 bits.
Although the above methods represent notable developments in the art of holographic interferometry, further developments and improvements are desired.